1. Field of the Invention
This invention relates to rubber compositions suitable for use in tread portions of automobile tires. More particularly, the invention is directed to the provision of such a composition comprised of a selected rubber combined with a selected type of carbon black of specified particle characteristics.
2. Description of the Prior Art
Tires for use in highly sophisticated sports cars should meet various physical and mechanical properties and driving stability in particular. An approach has been proposed to cope with this trend with the use of a tread rubber of high tan .delta. at 60.degree. C. This quality is closely associated with the gripping force of the tire on the road surface.
To attain increased tan .delta. at 60.degree. C., polymeric rubbers of high glass transition temperatures (Tg) have been used which are selected for instance from styrene-butadiene copolymer rubbers abundant in styrene content or in 1,2-vinyl content. Alternatively, large amounts of carbon black have been incorporated with a given starting rubber.
High Tg rubbers, however, are not wholly satisfactory as they are rather dependent on temperature, insufficiently hard and rigid at elevated temperature and inadequate in gripping force. Too much carbon black fails to get homogeneously dispersible, rendering the finished rubber composition susceptible to great hysteresis loss and hence objectionable heat buildup and poor mechanical strength.